Advanced composite sandwich structures have been widely used in aerospace structures, autos, armors, wind turbine blades, pipelines, bridge decks, etc. due to their superior structural capacity in carrying transverse loads with minimal weight penalty [1-4]. Although sandwich construction has been extensively used in various fields, sandwich panels have not been fully exploited in critical structural applications due to damage tolerance and safety concerns.
Sandwich structures typically consist of skins (surfacing and back plates) and a core. The skins are mainly responsible for carrying the bending moment (in blast protective sandwich panels, the surfacing plate is also responsible for eroding, breaking and slowing down the projectiles) and the core takes care of separating and fixing the skin, carrying the transverse shear load, providing impact resistance, and taking other functional duties. Numerous efforts have been made to explore high-performance sandwich panels over the past several decades.
For a typical sandwich structure, three elements dominate its performance and function: the face sheets, the core, and the bond between the core and the face sheet. A major problem of sandwich panels is the debonding at or near the core/face sheet interface, especially under impact loading, which can lead to a sudden loss of structural integrity and cause catastrophic consequences. Such debonding may also restrict the contribution of impact energy absorption by the core to the entire sandwich structure. Despite the efforts in previous studies, this problem has not been well addressed.
Various types of core materials have been studied such as foam core (polymeric foam, metallic foam, ceramic foam, balsa wood, syntactic foam, etc.) [3, 4], truss, honeycomb and other web cores [5], 3-D integrated core [6, 7], foam filled web core [7, 8], laminated composite reinforced core [9], etc. While these core materials have been used with a certain success, they are limited in one way or another. For example, the brittle syntactic foam core absorbs impact energy primarily through macro length-scale damage, sacrificing residual strength significantly [10-12]; and web cores often lack suitable bonding with the skin and also have impact windows [7, 8]. An impact window is the open space in a core, which allows easy perforation of projectiles or escape of anything (e.g., fluid) which may be contained behind the panel. Among the foam cores, metallic foam has also been developed. Metallic foam material has received rapid and intensive attention over the past decade due to its high specific stiffness and superior energy absorption ability [13, 14, 15].
Previously, it was found that by filling the empty bays formed by continuous fiber reinforced polymer grid skeleton with polymeric material, the resulting composite sandwich could be improved as to impact mitigation [16, 17, 18, 19], although these approaches continued to have important limitations.
Prior approaches did address impact mitigation by:
(1) each cell is a small panel or mini-structure with elastic boundary, it thus tends to respond to impact in a quasi-static manner, i.e., similar to the behavior under static load;
(2) the periodic grid skeleton, the primary load carrying component with 2-D continuity, could be responsible for transferring the impact energy elastically, dissipating the energy primarily through vibration damping and providing the in-plane tensile strength and in-plane shear resistance;
(3) the light-weight polymer matrix in the bay, the secondary load carrying component, could be primarily responsible for absorbing impact energy through damage;
(4) the grid skeleton and the polymer in the bay could develop a positive composite action, i.e., the grid skeleton confines the polymeric bay to increase its strength and the polymer matrix provides lateral support to resist rib local buckling and crippling. In addition, the polymeric bay could also provide additional in-plane shear strength for bi-grids such as orthogrid; and
(5) the core and skin could be fully bonded because the bay is fully filled, without the limitation of web cores.
However, with the prior approaches it is found that when the impact is on the rib or node of the grid skeleton, the residual strength is reduced considerably, due to the brittleness of the glass fibers [see, e.g., 17, 19]. For example, with about 300 J of impact energy, the projectile perforated the panel, suggesting poor perforation resistance [19]. Furthermore, because the impact caused fracture of the reinforcing fibers [19], and because the fibers are the primary load carrying component, this caused prior sandwich panels to lose their load carrying capacity permanently. Consequently such panels were radically impaired with subsequent impacts such as may occur in attack or military situations. Additionally, as the projectile impacted these materials it broke the reinforcing fibers, consequently the impact energy or impact wave could not be distributed or absorbed by the whole structure and this led to local perforation.
Another major problem of sandwich panels prior to the present invention has been debonding at or near the core/face sheet interface under impact loading, which can lead to a sudden loss of structural integrity and cause catastrophic consequences. The debonding may also restrict the contribution toward impact energy absorption by the core to the entire sandwich structure. This problem of debonding at or near the core/face sheet interface has remained an unmet need in the field.